Vehicle wheel bearing apparatus has advanced not only in reducing its manufacturing cost but in reducing its weight and size in order to improve fuel consumption. In these bearing apparatus, a wheel hub and a double row rolling bearing are formed as a unit. This is known as a representative example of a so-called third generation type. Here, each of the inner and outer members is formed with a flange. One of the inner raceway surfaces of the double row rolling bearing is formed directly on the wheel hub. The other inner raceway surface is formed on a separate inner ring which is press-fit onto the wheel hub.
FIG. 4 illustrates a vehicle wheel bearing apparatus which rotatably supports the wheel relative to the suspension apparatus of the third generation type of the prior art. An outer member 51, an inner member 54, including a wheel hub 52 and an inner ring 53 press-fit onto the wheel hub 52, and double row rolling elements 55 and 55, contained between the outer and inner members 51 and 54, form the bearing apparatus. The wheel hub 52 is integrally formed at one end with a wheel mounting flange 56. The flange 56 mounts a vehicle wheel (not shown) and has an inner raceway surface 52 on its outer circumferential surface. A cylindrical portion 52b axially extends from the inner raceway surface 52a. Hub bolts 57, to secure the wheel, are arranged, via an equidistant space, along the circumference of the wheel mounting flange 56. The inner ring 53, formed on its outer surface with an inner raceway surface 53a, is press-fit on the cylindrical portion 52b of the wheel hub 52. The inner ring 53 is secured on the wheel hub 52 by a caulked portion 52c. The caulked portion 52c is formed by plastically deforming the end of the axially extending cylindrical portion 52b radially outward to prevent the inner ring 53 from slipping off the wheel hub 52.
The outer member 51 is integrally formed with a body mounting flange 51b. The outer member 52 has double row outer raceway surfaces 51a and 51a on its inner circumferential surface. Double row rolling elements 55 and 55 are freely rollably contained between the double row outer raceway surfaces 51a and 51a and the opposed inner raceway surfaces 52a and 53a. 
The wheel hub 52 is made by forging carbon steel, including carbon of 0.40˜0.80% by weight, and surface hardening it by high frequency induction quenching in a region from the base of the wheel mounting flange 56 to the cylindrical portion 52b through the inner raceway surface 52a. However, the caulked portion 52c remains in its rare condition as having its surface hardness after forging. On the other hand, the inner ring 53 is made of high carbon chrome bearing steel such as SUJ 2 and is hardened to its core by quenching.
Accordingly it is possible to realize a low cost vehicle wheel bearing apparatus that has a sufficient durability to prevent a generation of damages such as cracks at the caulked portion 52c. A through bore 58 is formed in the center of the wheel hub 52 which also realizes weight reduction of the wheel hub 52 and reduction in manufacturing cost by commonly using the wheel hub 52, after forging between structures of the bearing apparatus, to support a driving wheel and to support a driven wheel. See, Japanese Laid-open Patent Publication No. 2002-61660.
In the prior art vehicle wheel bearing apparatus described above, since the wheel hub 52 is formed with the through bore 58 at the center of the wheel hub 52, it is possible to reduce the weight of the wheel hub 52. The through bore 58 enables common use of the wheel hub 52, after forging, between the bearing apparatus to support a driving wheel and to support a driven wheel. On the contrary, the through bore 58 in the wheel hub 52 also causes several problems such as reduction of rigidity of the wheel hub 52 especially at a region on the inboard side of the wheel hub 52 where a large moment load is applied. Thus, this reduces the strength and durability of the wheel hub 52.
If the rigidity of the cylindrical portion 52b is small, the cylindrical portion 52b near the caulked portion 52c would be deformed radially outward during the caulking process. Accordingly, the inner ring 53 would also be deformed radially outward and thus hoop stress is generated within the inner ring 53 which reduces the durability of the inner ring 53.
In addition, this kind of bearing apparatus is usually subjected to severe external environments above a road surface and frequently suffers from exposure to muddy water. Especially in coastal regions, the bearing apparatus is exposed to salt water. Further, in cold environments, it is exposed to muddy water which includes salt or anti-freezing agents. Since the inner ring 53 is exposed to these conditions, as a naked steel surface, it would become rusty. If the generation of rust progresses in the inner ring 53, it sustains a large hoop stress. Diffusive hydrogen present in the environment enters into the metallographic structure of the inner ring 53 and causes a so-called “delayed fracture” causing the hydrogen embrittlement which causes destruction of the metallic grain boundary.